1. Field of the Invention
This invention relates generally to a unipolar phase DC brushless motor and, more particularly, it relates to a staggered pulse width modulation (PWM) apparatus and method for electromagnetic interference (EMI) minimization in a unipolar phase DC brushless motor.
2. Background of Related Art
In any four (4), or five (5) phases, of a unipolar brushless DC motor, only two of the motor phases are energized at any given time. For instance, in a four (4) phase motor (essentially a two phase motor), only two motor phases are energized at any given time resulting in only fifty (50%) percent of the copper being utilized at any given time during operation of the motor. Likewise, in a five (5) phase motor, only two (2) motor phases are energized at any given time during operation of the motor.
FIG. 24 is a graph illustrating the conventional technique of simultaneously energizing the active phases of a unipolar brushless DC motor when pulse width modulation (PWM) is used.
In particular, since the supply current ripple of the motor is proportional to the current in the two (2) motor phases which are switched on simultaneously, there is a significant contribution to high frequency electromagnetic interference (EMI) on the motor supply leads. The electromagnetic interference is typically due to noise caused by the rate of change in supply current (di/dt) in the high power stage of a printed circuit board (PCB) where high inductance exists. Generally, high impedance traces exist because of space limitations placed on designers to provide a high power electronic motor package in minimal packaging space.
FIGS. 25A to 25D show a modulation of the width of each of the pulses within the activation phase shown in FIG. 24. In particular, FIGS. 25A and 25B show the same pulsed signals in an activation phase, with each of the pulsed signals having a duty cycle of less than 50%. FIGS. 25C and 25D show pulsed signals having a duty cycle of greater than 50%.
Conventional techniques have attempted to minimize electromagnetic interference from motors. For instance, U.S. Pat. No. 5,440,214 to Peeters (hereinafter `Peeters`) discloses drive control and interface apparatus for driving a stepper motor. The drive control and interface apparatus of Peeters attempts to minimize possible electrical noise spikes by turning the drives on and off with a fixed delay between the turn on and turn off times. Unfortunately, Peeters does not address minimization of noise harmonics due to variations in line current when two phases are turned on, nor when the current in both coils increases simultaneously.
U.S. Pat. No. 5,666,042 to Lewis discloses a switching current spike limiter for a three phase coupled inductor conductive battery changer. The current spike limiter of Lewis includes a PWM driver which boosts the voltage by pulse width modulating the lower switches in a concurrent or staggered fashion. Unfortunately, Lewis fails to disclose details regarding the staggering, e.g., whether the staggering is conducted with a fixed or variable delay, nor the intended purpose of the staggering of the pulses. Furthermore, the current spike limiter of Lewis relates specifically to a battery charger, not to a drive for a motor.
Accordingly, there exists a need for providing a motor drive which minimizes EMI in a unipolar or bipolar phase DC brushless motor. Moreover, there is a need for providing a motor drive which minimizes noise harmonics caused by an increase in the supply current during the time the current in two coils is increasing.